Catalysts for the preparation of an alkenyl carboxylate from an olefin, a carboxylic acid and oxygen are known in the art. Such catalysts are based on a Group 8 metal as a catalytically active metallic species on a carrier. The preparation of the catalysts is well documented.
For example, the process for preparing the catalyst of U.S. Pat. No. 4,048,096 comprises the steps of selecting a carrier, precipitating a Group 8 metal compound into the carrier, converting the precipitated Group 8 metal compound into metallic species, and subsequently purifying the catalyst by washing with water.
Similar schemes for the preparation of the catalysts are known from U.S. Pat. Nos. 5,179,057 and 5,189,004. The latter documents highlight the removal from the catalysts of sodium ions, which are introduced during the catalyst preparation, for example as a portion of a precursor of the precipitated Group 8 metal compound (for example sodium tetrachloropalladium (II)), or as a portion of the precipitating agent which is used for precipitating the Group 8 metal compound (for example a sodium silicate or sodium hydroxide). Both documents teach that the removal of sodium ions leads to an increase in the activity of the catalyst and to a decrease in the selectivity when the catalyst is used in the process for the preparation of an alkenyl carboxylate.
U.S. Pat. Nos. 5,250,487 and 5,422,329 relate to Group 8 metal catalysts for use in a process for the preparation of an alkenyl carboxylate from an olefin, a carboxylic acid and oxygen. The catalysts are based on carrier particles which have been pressed with the aid of a binder of one or more salts of carboxylic acids. The carrier particles are washed with an acid for the removal of the cations of the binder from the support particles. The acid may be a mineral acid such as hydrochloric acid, sulfuric acid, phosphoric acid, or nitric acid. If the anion of the acid is detrimental to the catalyst, such as chloride and sulfate anions, excess acid may be removed by washing out with distilled water. If the salts employed during the subsequent catalyst preparation contain constituents harmful to the catalyst, such as chloride or sulfate, the catalyst is washed with water.
Thus, in the prior art documents relating to the Group 8 metal catalysts attention has been paid to the detrimental effects of certain catalyst impurities which are introduced intentionally during the carrier or catalyst preparation and to the removal of these impurities. Such impurities may be removed by using dedicated means. For example, the cations of the binder are removed by washing with acid.
WO-00/15333 teaches the preparation of improved catalysts, in particular silver impregnated alumina based catalysts for the vapor phase production of epoxides. The improvement is achieved by lowering the concentration of ionizable species present on the surface of the carrier, in particular by treating the carrier by washing with boiling water prior to catalyst preparation.
Although the known Group 8 metal catalyst have appreciable activity and selectivity in the preparation of alkenyl carboxylate from an olefin, a carboxylic acid and oxygen, further improvements of these catalysts are desirable, in particular in their ability to maintain their level of activity and selectivity during use over a long period of time.